The fusion drive is in the pc world a hybrid drive. Conventional hard drive platters with a ssd portion of memory on the controller board. You can't simply swap out the Chip and attach a nmve m2... Your only option is to purchase a bigger hybrid hard drive..
Stop using the computer, turn it off. Take it to a computer shop so they can use recovery software to get whatever is left.. If your data you lost is important there is no simple way to get it back. Software recovery is never 100%
A more detailed question should be asked. What type of engine. Is the check engine light on. When is the last time you had a tuneup? My guess is never... Double the fuel consumption you either have drag (issue with the brake system, seized calipers), sensor(s), fouled plugs, air filter clogged, vacuum leaks, a fuel leak, under inflated tires. Although it won't be a tire issue.. My guess is start with a tuneup, plugs, filter etc..